Acromioclavicular joint dislocation occurs as a result of an acute traumatic event. Patients are most often young and active in sports. The incidence of acromioclavicular joint separation varies with the sportive activity and can reach up to 20 % in skiing. Altogether, separation of the acromioclavicular joint accounts for 4–6 % of all joint dislocations [1]. Injuries of the shoulder complex are associated with a decreased range of motion and may be followed by serious consequences when diagnosis is delayed or even wrong.

When the patient has suffered from a traumatic event, a direct blow to the adducted arm is frequently described. Bruises and cranial dislocation of the lateral clavicle are recognized. A neurovascular exam of the involved extremity is important and reducibility of the joint with upward pressure of the elbow stabilizing the clavicle superiorly can be performed (note that the shoulder drops down, the clavicle does not move up).

Using X-rays, a.p. projection of the glenohumeral joint as well as Y-view or transaxillary views are taken. To evaluate the acromioclavicular joint directly, the Zanca view should be used by tilting the center-beam 30–45° caudocranial, aiming on the acromioclavicular joint (Zanca 1971). The Alexander view (Y-view with maximum arm adduction) helps to verify subluxation in acromioclavicular joints (Alexander 1954). If a full dislocation is suspected, a.p. projections comparing both sides should be taken, using 5–10 kg weights. Transaxillary views are highly recommended to diagnose a horizontal instability. Ultrasound offers a good diagnostic tool for degenerative changes as well as lower grade injuries of the acromioclavicular joint (Rockwood I–II). In higher-grade lesions (Rockwood IV–VI), muscle hematoma and ruptured muscle insertions can be diagnosed. The distance from the coracoid process to the clavicle can be measured and compared to the healthy contralateral side. The results correlate positively with X-ray findings [10, 11]. CT scan and scintigraphy (arthritis) can be performed but are of lesser importance than an MRI. Early osteolysis (repetitive trauma) or rheumatoid arthritis can be diagnosed through MRI. In acute acromioclavicular dislocations, a huge number of missed injuries (15 % SLAP lesion, 5 % fractures, 4 % rotator cuff tears) were found [12]. Acromioclavicular separations are classified by Tossy (1963) and Rockwood (1984). Kraus et al. published a new measurement tool for instability of the acromioclavicular joint, the acromioclavicular joint instability score (ACJI), in 2010.

Treatment strategies vary and controversy remains regarding the optimal course of action. There are more than 150 different conservative and operative treatment options to stabilize the joint [2]. There is little literature about long-term results, even for well-established treatment strategies. Likewise, no long-term results for newly developed operative procedures are available [3–7]. At this point, there is no therapeutic gold standard for acromioclavicular joint dislocation. Typically, a surgical indication is made with higher-grade acromioclavicular separations. Rockwood III grade injuries must be evaluated individually.